Rectification column for isolating pure substances from high boiling air-and/or temperature sensitive substances

ABSTRACT

A reflection column which contains liquid distributors having at least 500 drip points/m 2 , which are arranged at an angle of about 90° C. to cloth layers of packing elements located immediately therebelow; and a combination of insulation and protective heating. The column is advantageously used for the rectification of mixtures of high-boiling air- or temperature-sensitive substances requiring effective separation efficiency.

This is a Division of application Ser. No. 08/981,491, filed on Jan. 8,1998, U.S. Pat. No. 6,111,117 which is a continuation of internationalapplication PCT/EP 96/02852, filed Jun. 29, 1996.

The invention relates to a process for isolating pure substances frommixtures of high-boiling air- and/or temperature-sensitive substanceswhich require a high separation efficiency by rectification under mediumvacuum, in particular a process for the rectification of crude vitamin Eacetate, for the purpose of purifying from lower-boiling andhigher-boiling impurities, and to columns suitable for this process.

FIELD OF THE INVENTION

The workup of product mixtures by distillation generally affords thebest results in the case of countercurrent distillation (also calledrectification), ie. a specific distillation process with acountercurrent of reflex flowing downwards and vapor flowing upwards inrectification columns. The rectification columns normally used for thispurpose are such that the mixture to be separated is introduced into themiddle part of the column, and the vapor on its way through the columnfrom the bottom to the top becomes enriched in more volatile componentswhile the reflux from top to bottom becomes enriched in less volatilecomponents. Mass and heat transport is intensified by elements fittedinside the column, such as column plates or packings, which ensure asufficient contact time of the phases and a sufficiently large phaseboundary area. However, these column internals, together with the refluxflowing downwards, result in a resistance in the column which is calledthe pressure drop. The pressure drop in a column depends not only on thenature and amount of the compounds to be rectified but also verystrongly on the nature of the column internals.

Generally used for the fractionation of high-boiling substance mixtureswhich require a high separation efficiency are rectification columnswhich have packings which are built up systematically in regulargeometry and have defined areas for the countercurrent phases to passthrough, because packings with a regular structure are distinguished incomparison with random packings by the possibility of higer flow ratesand a better separation effect, and have a lower specific pressure dropand a smaller required packing volume and thus also a smaller necessarymass and heat exchanging height. They are therefore used in all vacuumrectifications in which, because of the temperature-sensitivity of themixture to be separated, it is particularly important to limit thepressure drop in the column.

Particularly suitable column packings are metal cloth packings of the BXand CY types supplied by Sulzer (cf. Sulzer company publication“Trennkolonnen für Distillation and Absorption”) and metal clothpackings with a similar effect supplied by other companies such as MontzGmbH.

A diagrammatic representation of such columns is to be found, forexample, on page 103 of the textbook “Thermische Trennverfahren” byKlaus Sattler, VCH Verlagsges.mbH, Weinheim (FRG), 1988. Concerningfurther details of the rectification of substance mixtures, we refer tothis textbook by Klaus Sattler, pages 101-225, in particular 120-160 and199-214.

The highest product temperature occurs in the bottom of a column. Apartfrom the overhead pressure, it is closely determined by the pressuredrop from the column internals derived from the required separationefficiency. In rectification columns, the bottom temperature is notsignificantly affected by reducing the overhead pressure to less than0.5 mbar.

The thermal stressability of many high-boiling mixtures is so low that,despite the use of the described metal cloth packings with orderedstructure and overhead pressures in the column of only required for thenecessary separation efficiency would result in bottom temperatureswhich are above the decomposition range for the compounds to beseparated. This is why to date the fractionation of such mixtures bydistillation has generally been carried out in the high vacuum range(about 10⁻¹ to 10⁻⁵ mbar), ie. short-path distillations or moleculardistillations are used. However, in the case of mixtures with lowrelative volatilities it is possible to obtain high purities only withlow yields in these distillations.

One example of a mixture of high-boiling and highly air- and/ortemperature-sensitive substances which require a high separationefficiency is synthetic vitamin E acetate (VEA) which is preparedindustrially by reacting trimethylhydroquinone with phytol or isophytoland subsequently esterifying with acetic anhydride, and still containssmall amounts of colored lower-boiling and higher-boiling impurities.Since VEA is increasingly being used in the human diet and for healthprophylaxis, the demands on the purity of this product are increasing.Rectification, which is very advantageous in general for purification ofproducts on an industrial scale, is very difficult with VEA owing to itshigh boiling point together with its instability at higher temperatures.This is why to date essentially distillations under high vacuum or evenmolecular distillations have been carried out in order to be able todistill VEA at the lowest possible temperatures.

Despite the use of high vacuum (10⁻¹ to 10⁻⁵ mbar), the puritiesobtained in the prior art are generally only 97.3% (cf. DE 2 743 920),98% (cf. DE 42 08 477 and JP-B-58 011 869), 98.5% (cf. U.S. Pat. No.3,459,773) or 98.5 to 99% (cf. DE 2 160 103). Purities above 99% havebeen obtained only by molecular distillation, namely purities of 99.3%according to JP-A 51/14671 and 99.5% according to JP-A-62/226976,although it should be noted that the products obtained in this way wouldpresumably show lower purities on investigation with the more accurateanalytical methods in use now and with purer comparison substances. Inaddition, the yields which can be obtained in such distillations are ineach case rather low.

However, since distillations under high vacuum, but especially moleculardistillations, while giving high purities not only have the disadvantageof low distillation yields but are also extremely costly in terms of thecapital costs and in terms of the operating costs, it was object of theinvention to develop a process for the separation by distillation ofhigh-boiling air- and/or temperature-sensitive substances which requirea high separation efficiency, in which high-vacuum distillation ormolecular distillation is unnecessary, ie. a process in which anoverhead pressure of from 0.1 to 2 mbar is sufficient and thusconsiderably less costly pressure-reducing processes are necessary and ahigh distillation yield is obtained.

It was an object of the invention in particular to find a process forthe final purification of VEA by distillation able to result incolorless VEA with a primary of 99% or more and good distillation yieldseven by rectification under medium vacuum in columns containing metalcloth packings with ordered structure.

The invention thus relates to a process for isolating pure substancesfrom mixtures of high-boiling air- and/or temperature-sensitivesubstances which require a high separation efficiency by rectificationunder medium vacuum in columns containing metal cloth packings withordered structure, which comprises carrying out the rectification in amass transfer column in which

a) the liquid distribution is undertaken with channel distributors with500 or more drip points/m², preferably 900 to 1200 drip points/m²,

b) in which the channels of the distributors are arranged at an angle ofabout 90° to the cloth layers of the packing elements locatedimmediately below the distributor,

c) in which 2 or more packing elements which have a height of from 20 to100 mm and whose cloth layers are in each case rotated by 90° withrespect to one another are located immediately below the liquiddistributors,

d) the column is designed so that virtually no heat exchange through thecolumn wall can take place during the rectification, and

e) for air-sensitive substances, the column is designed so that it ispossible to operate virtually with exclusion of air.

The process according to the invention takes place particularlyadvantageously when the cloth packings used are such that the angle ofinclination of the serration of the individual cloth layers of thepacking to the column axis is as small as possible in order to minimizethe specific pressure drop of the packing.

Whereas the angle of inclination of the serration is generally 30° withthe usual Sulzer BX packings, the cloth packings preferred in theprocess according to the invention are those whose angle of inclinationof the serration of the cloth layers to the column axis is from 0 to25°, preferably 3 to 10°. The angle of inclination of the serration ofthe cloth layers to the column axis is illustrated in FIG. 1. In thisFigure, 1 means the angle of inclination of the serration of the clothlayers to the column axis and 2 means cloth layers.

The process according to the invention is particularly important for thefinal purification of VEA, ie. in the case where the mixture ofhigh-boiling air- and/or temperature-sensitive substances rectified is aVEA contaminated with colored, lower being and higher-boilingsubstances.

Taking the example of the purification of VEA, the developments inapparatus and process technology of the process according to theinvention are to be explained in detail hereinafter.

Columns can be operated industrially at a cost which is still acceptablewith overhead pressures of from 0.5 mbar. At a pressure of 1 mbar, VEAhas a boiling point of about 240° C. The limited thermal stability ofVEA means that the temperature at the bottom of the column is limited toabout 260-270° C. A rectification column must therefore be operated witha maximum bottom pressure of only 4 mbar. This means that a pressuredrop of only about 3-3.5 mbar between top and bottom of the column canbe tolerated. This is very difficult to achieve because a separationefficiency of about 10 to 30 separation stages is necessary for finalpurification of the VEA, and a pressure drop of from 0.3 to 0.5 mbar perseparation stage must normally be expected. In the operating rangesaccording to the invention there were no measurement available on thepressure drops.

According to feature a) of the main claim, a liquid distribution withchannel distributors with 500 or more drip points is claimed.Distributors which are similar but circular and are also calledcapillary distributors are marketed by the companies Sulzer and Montzand are described, for example, in EP 512 277. Known channeldistributors generally have only 50 to 60 drip points per m².

The use according to the invention of the channel distributors bringsabout a reduction in the pressure drop in 2 different ways. They result,on the one hand, in a rapid and extremely fine distribution and thuseventually in better utilization of the packing for distribution of themixture to be separated and, on the other hand, in a very low tricklingdensity. The lower limit of liquid flow rate stated for Sulzer packagingof the BX type is about 0.2 m³/m^(2·)h. Through the use, according tothe invention, of the channel distributors with 500 or more, preferably900 to 1200, drip pints with VEA the liquid flow rates achieved in theprocess according to the invention are only 0.03-0.3 m³/m^(2·)h overheadand 0.03-1.0 m³/m^(2·)h in the stripping part of the column. It has beenfound, surprisingly, that even with such low liquid flow rates thecomplete wetting of the metal packings, which is required for optimalseparation efficiency, is ensured. This low trickling density means thatthe gas flow rate in the column and thus the pressure drop is extremelylow.

However, to obtain an optimal separation efficiency it is important notonly to have a large number of drip points but also to arrange thedistributors having regard to the packing elements.

One layer of a cloth packing generally consists of a multiplicity of,usually single, cloth layers which are 170 mm high. Each packing layeris, when fitted, rotated by in each case 90° in relation to the previouslayer. The distributors are likewise arranged rotated by 90° in relationto the packing element located immediately below the distributors, or tothe packing layer located there.

The liquid now spreads out on one of these cloth layers at a particularangle. After an inflow length which depends on the spreading angle andthe distance between two drip points, a uniform film has formed over acloth layer.

Optimal utilization of the packing, ie. the fastest possibledistribution of the liquid on all cloth layers, is achieved when thepacking is rotated by 90° at this point.

Hence, according to the invention, 2 or more packing elements with aheight of from 20 to 100 mm, preferably 25 to 50 mm, in particular 35 to45 mm, whose cloth layers are in each case rotated by 90° with respectto one another are used underneath the liquid distributors. Theseparation of the packing into elements of small height means that thefastest possible distribution and thus optimal utilization of thepacking for the separation can be achieved. By contrast, in theconventional arrangement of packing, the inflow length is about 340 mm,which means that with a packing height of 2 meters about 17% of thepacking are not fully utilized for the actual separation operation.

According to feature e), the rectification is to be carried outvirtually with exclusion of air.

Laboratory tests have shown that, for example, in the case of VEA at thehigh temperatures required for the rectification even the slightestleaks in the distillation equipment result in the product darkening incolor, which cannot be tolerated because of the high demands on quality.The use of newly developed, particularly high quality sealing materialssuch as Helicoflex® supplied by Cefilac for sealing flanges and/oropenings for devices to monitor the process is therefore absolutelynecessary. It is particularly advantages to seal flanges by using weldedlip seals as described, for example, in German Patents DE 27 10 859, DE39 12 478 or DD 44 07 728.

As already explained, only small mass flows circulate in the mediumvacuum rectification columns according to the invention. Thus every lossof heat immediately leads to uncontrolled condensation on the columnwall, which reduces the separation efficiency of the column. Thepreventing of heat exchanges through the column wall can best be ensuredby a combination of insulation and protective heating of the column.

Such protective heating is advantageously achieved industrially in thefollowing way: a metal plate jacket is attached on a first insulatinglayer on the column jacket. This metal plate jacket is insulated again.Another metal plate jacket and the heating are finally insulated towardthe outside. The heating is then controlled in such a way that thetemperature difference between the column jacket and the first metalplate jacket is zero.

For the final purification of VEA according to the invention it isnecessary to operate the rectification with overhead pressures of from0.2 to 1 mbar, preferably 0.5 to 1 mbar, and with bottom pressures offrom 1 to 4 mbar, preferably 1.5 to 3.5 mbar, in particular 2 to 3 mbar.

In the rectification of VEA according to the invention, the packedcolumns are operated with a liquid flow rate of from 0.03 to 0.3m³/m^(2·)h in the concentrating part of the column and with a liquidflow rate of from 0.03 to 1.0 m³/m^(2·)h in the stripping part of thecolumn.

In general, and also for VEA, 2 columns are necessary to remove coloringlower-boiling impurities and higher-boiling impurities. FIGS. 2a and 2 bdepict diagrammatically 2 possible distillation concepts for therectification of VEA in 2 packed columns. The meanings in these are

1 inlet for crude vitamin E acetate

2 outlet for feed quality

3 outlet for feed quality+residue

4 outlet distillate

5 outlet for food or drug quality, ie. purity>99%

6 bottom stream

7 rectification to remove high boilers

8 rectification to remove low boilers.

However, it is also possible to use equivalent distillation facilitiessuch as a baffle plate column. A baffle plate column is, in general, acombination of 2 separate column packings within an outer column jacketon which inlet and side discharge are located.

Such a baffle plate column is depicted diagrammatically in FIG. 2c. Inthis, 1, 2, 3 and 5 have the same meanings as in FIGS. 2a and 2 b, and 9means a baffle plate column.

The invention also relates to the packed columns equipped for theprocess according to the invention and to the use thereof for the finalpurification of impure VEA by rectification in the pressure range from0.1 to 2 mbar.

These are packed columns with a stripping part and concentrating partcontaining metal cloth packings with ordered structure, each of whichare rotated by 90° with respect to one another, for the rectification ofmixtures of high-boiling air and/or temperature-sensitive substanceswhich require a high separation efficiency, with overhead pressures offrom 0.1 to 2 mbar, wherein,

a) they contain channel distributors with 500 or more drip points/m² asliquid distributors, preferably 900 to 1200 drip points/m²,

b) the channels of the liquid distributors are arranged at an angle of90° to the cloth layers of the packing elements located immediatelybelow the distributor,

c) they contain 2 or more packing elements which have a height of from20 to 100 mm and whose cloth layers are in each case rotated by 90° withrespect to one another below the liquid distribution,

d) they contain a combination of insulation and protective heating,which ensures that virtually no heat loss through the column wall cantake place during the rectification, and

e) the seals and flanges are designed so that virtually exclusion of airis ensured.

A packed column which can be used according to the invention for therectification of VEA is depicted diagrammatically in FIG. 3.

The meanings in this are:

1 inlet for cooling medium

2 condensor

3 outlet for cooling medium

4 distillate

5 reflux

6 channel liquid distributors with 500 to more drip points/m²

7 packing elements with a height of from 20 to 100 mm

8 packing elements with a height of about 170 mm

9 a combination of insulation and a protective heating

10 inlet for crude VEA

11 liquid collector

12 side discharge

13 sealing element

14 outlet for heating medium (exit)

15 falling film evaporator

16 inlet for heating medium (entry)

17 bottom discharge

18 flanges

19 seals

It is possible with the aid of the packed columns according to theinvention and of the process according to the invention to rectifymixtures of high-boiling air- and/or temperature-sensitive substanceswhich require a high separation efficiency, such as crude VEA ormixtures of tocopherols with impurities, also under medium vacuum, ie.pressures of from 0.2 to 2 mbar, preferably 0.5 to 1 mbar, in columnswith cloth packings with low pressure drop and with very gooddistillation yields.

Description, by way of example, of the process for purifying crudevitamin E acetate by the distillation concept shown in FIG. 2a.

Degassed, synthetic crude vitamin E acetate with a content of about 96%VEA is introduced at from 200 to 250 degrees Celsius through the inlet10 into the middle of the first rectification column containing a heightof from 3 to 5 m of packings of the Sulzer BX of Montz A3 type. Theliquid is distributed uniformly over the cross-section of the column atthe top and at the inlet to the column by the newly developedhigh-efficiency channel distributors 6. The packing elements 7 with aheight of from 20 to 100 mm are located underneath the newly developedchannel distributors. The cloth layers of the packing elements arrangedimmediately below the channel distributors 6 are rotated by an angle of90° with respect to the channels of the liquid distributors. The columnis equipped with a protective heating 9 and is operated adiabatically.Flanges and connections are provided with welded lip seals or highquality metal seals.

The overhead pressure of the column is from 0.5 to 1 mbar. The refluxratio is from 2 to 4. The liquid flowing back in the column is at from170 to 220 degrees Celsius. From 5 to 20% of the inlet stream areremoved at the bottom of the column.

The overhead product at from 200 to 250° C. from the first column isdelivered into the middle of the second rectification column containinga height of from 3 to 5 m of packings of the Sulzer BX or Montz A3 type.The liquid is distributed over the column cross-section at the head andat the inlet by the newly developed high efficiency channeldistributors. This column is also equipped with a protective heating andis operated adiabatically. Flanges and connections are provided withwelded lip seals or high quality metal seals.

The overhead pressure of the column is about 0.5 mbar. The reflux ratiois from 5 to 15. The liquid flowing back into the column is at from 170to 220° C. From 5 to 20% of the inlet stream are removed at the top andbottom of the column. The vitamin E acetate removed in the sidedischarge directly above the vaporizer of the second column is virtuallycolorless (color number less than 2) and is of drug quality with apurity of more than 99%. The yield of product with this purity is about50% of theory.

The residue taken from the bottom of the first rectification column isseparated in a thin-film evaporator under a pressure of from 1 to 5mbar. The distillate obtained in this way can, like the overhead andbottom products from the second column, be sold as vitamin E acetate foranimal feed purposes (feed quality). These products generally containfrom 90 to 99% VEA. The total distillation yield of feed and dropquality VEA is from 95 to 98%.

We claim:
 1. A rectification column which comprises: a) channel liquiddistributors with at least 500 drip points/m² as liquid distributors; b)the channel liquid distributors being arranged at an angle of about 90°C. to cloth layers of packing elements located immediately below thechannel liquid distributors; c) 2 or more of the packing elementsimmediately below the channel liquid distributors which have a height offrom 20 to 100 mm and whose cloth layers are in each case rotated by 90°with respect to one another; d) a combination of insulation and aprotective heating, which ensures that virtually no heat loss throughthe column wall occurs during rectification; and e) seals and flangeswhich are designed to virtually exclude air in the rectification column.2. The column of claim 1, wherein the liquid distributors comprises 900to 1,200 drip points/m².
 3. The column of claim 1, wherein the packingshave an angle of inclination of the serration of the layers to a columnaxis of from 0 to 25°.
 4. The column of claim 3, wherein said angle ofinclination is from 3 to 10°.
 5. The column of claim 1, wherein theseals are welded lip seals.
 6. The column of claim 1, wherein said 2 ormore packing elements have a height of from 25 to 50 mm.
 7. The columnof claim 6, wherein said 2 or more packing elements have a height offrom 35 to 45 mm.
 8. The column of claim 1, which is adiabaticallyoperable.
 9. The column of claim 1, wherein said protective heating iseffected by a first metal plate jacket which is attached on a firstinsulating layer on a jacket of the rectification column, wherein saidmetal plate jacket is also insulated by a second insulating layer, andanother metal plate jacket and a heating device are attached to thesecond insulating layer, and the heating device is insulated toward theoutside, whereby during heating a temperature difference between thecolumn jacket and the first metal plate jacket is zero.